The present invention relates to acoustic projectors, more particularly to acoustic projectors for producing pressure waves in fluidic environments, such as may be suitable for underwater sonar applications.
Various types of acoustic projectors have been used for producing pressure waves in fluids, including those based on piezoelectric and electromagnetic (EM) principles. Piezoelectric projectors tend to be fragile due to their ceramic construction, and require power conversion electronics (which can be heavy, bulky and costly) to permit high-voltage-low-current powering such as delivered by batteries and some power amplifiers. The so-called “boomer” electromagnetic projectors, used in oil and gas exploration, can only be used at or just beneath water surfaces, and generate sounds that reach water bottoms at much reduced levels due to spreading losses. A conventional “boomer” EM projector avails itself of the low atmospheric pressure in order to produce a large cavitation bubble that collapses, thereby producing a transient underwater pulse the level of which decays in amplitude in accordance with 1/r2, where r is the distance in meters away from the source.
The present inventors previously disclose, in their U.S. Pat. No. 6,570,819 B1, issued 27 May 2003, entitled “Low Frequency Acoustic Projector,” incorporated herein by reference, an electromagnetic (EM) acoustic projector that produces pressure waves in fluids. As their '819 acoustic projector is typically embodied two pancake-shaped spiral-wound, electrically conductive wire coils (made, e.g., of copper) are spaced slightly apart (e.g., ⅛ inch or less), parallel to each other. Each spiral-wound wire coil is encapsulated in a shell (made, e.g., of a fiberglass resin). A ring (made, e.g., of an epoxy) encircles the planar junction between the two encapsulated spiral-wound wire coils, and is flexibly attached to each encapsulated spiral-wound wire coil via, for instance, plural flexible bands (made, e.g., of a rubber material such as silicon or urethane). Voltage is applied across one end of each spiral-wound wire coil so that currents of opposite polarities flow in the two spiral-wound wire coils. The resultant magnetic forces cause the two encapsulated spiral-wound wire coils to repel each other (when current flows in the coils) and to come back to each other (when current does not flow in the coils) in accordance with the periodicity of the electrical excitation. For a sinusoidal electrical excitation of given drive frequency, the resultant oscillation (vibration) is at a frequency that is twice the drive frequency, since the magnetic forces are always repulsive regardless of the polarity (i.e., in negative sinusoidal cycle versus in positive sinusoidal cycle) of the sinusoidal electrical excitation. The flexible bands together with the hydrostatic water pressure maintain proximity of the coils, exerting restorative forces when current does not flow in the coils.
The above-described EM acoustic projector of U.S. Pat. No. 6,570,819 is quite useful but tends to be limited at greater water depths, e.g., at depths below twenty feet. The amount of power required for operation of a '819 acoustic projector will tend to increase, in accordance with increasing water depth, toward an impractical limit. That is, the '819 acoustic projector's effectiveness will be delimited by a prohibitive water depth, at or below which the '819 acoustic projector's electromagnetic resonance will become insufficient to overcome the increased hydrostatic pressure concomitant with the increased water depth.